Chlorine dioxide (ClO2) is a neutral compound of chlorine in the +IV oxidation state. It disinfects by oxidation; however, it does not chlorinate. It is a relatively small, volatile, and highly energetic molecule, and a free radical even in dilute aqueous solutions. Chlorine dioxide functions as a highly selective oxidant due to its unique, one-electron transfer mechanism in which it is reduced to chlorite (ClO2−). The pKa for the chlorite ion/chlorous acid equilibrium, is extremely low (pH 1.8). This is remarkably different from the hypochlorous acid/hypochlorite base ion pair equilibrium found near neutrality, and indicates that the chlorite ion will exist as the dominant species in drinking water.
One of the most important physical properties of chlorine dioxide is its high solubility in water, particularly in chilled water. In contrast to the hydrolysis of chlorine gas in water, chlorine dioxide in water does not hydrolyze to any appreciable extent but remains in solution as a dissolved gas.
The traditional method for preparing chlorine dioxide involves reacting sodium chlorite with gaseous chlorine (Cl2(g)), hypochlorous acid (HOCl), or hydrochloric acid (HCl). The reactions are:2NaClO2+Cl2(g)→2ClO2(g)+2NaCl  [1a]2NaClO2+HOCl→2ClO2(g)+NaCl+NaOH  [1b]5NaClO2+4HCl→4ClO2(g)+5NaCl+2H2O  [1c]Reactions [1a] and [1b] proceed at much greater rates in acidic medium, so substantially all traditional chlorine dioxide generation chemistry results in an acidic product solution having a pH below 3.5. Also, because the kinetics of chlorine dioxide formation are high order in chlorite anion concentration, chlorine dioxide generation is generally done at high concentration (>1000 ppm), which must be diluted to the use concentration for application.
Chlorine dioxide may also be prepared from chlorate anion by either acidification or a combination of acidification and reduction. Examples of such reactions include:2NaClO3+4HCl→2ClO2+Cl2+2H2O+2NaCl  [2a]2HClO3+H2C2O4→2ClO2+2CO2+2H2O  [2b]2NaClO3+H2SO4+SO2→2ClO2+2NaHSO4  [2c]At ambient conditions, all reactions require strongly acidic conditions; most commonly in the range of 7-9 N. Heating of the reagents to higher temperature and continuous removal of chlorine dioxide from the product solution can reduce the acidity needed to less than 1 N.
A method of preparing chlorine dioxide in situ uses a solution referred to as “stabilized chlorine dioxide.” Stabilized chlorine dioxide solutions contain little or no chlorine dioxide, but rather, consists substantially of sodium chlorite at neutral or slightly alkaline pH. Addition of an acid to the sodium chlorite solution activates the sodium chlorite, and chlorine dioxide is generated in situ in the solution. The resulting solution is acidic. Typically, the extent of sodium chlorite conversion to chlorine dioxide is low and a substantial quantity of sodium chlorite remains in the solution.
WO 2007/079287 teaches that the contamination of chlorine dioxide solutions with alkali metal salts accelerates decomposition of aqueous chlorine dioxide solutions. WO 2007/079287 further discloses a method of preparing a storage-stable aqueous chlorine dioxide solution, wherein the solution contains about 2500 ppm or less of alkali metal salt impurities. Alkali metal salt impurities disclosed are sodium chloride, magnesium chloride, calcium chloride and sodium sulfate.
Chlorine dioxide is known to be a disinfectant, as well as a strong oxidizing agent. The bactericidal, algaecidal, fungicidal, bleaching, and deodorizing properties of chlorine dioxide are also well known. Therapeutic and cosmetic applications for chlorine dioxide are known.
For example, U.S. Pat. No. 6,287,551 discusses the use of chlorine dioxide solutions for the treatment of Herpes virus infection. U.S. Pat. No. 5,281,412 describes chlorite and chlorine dioxide compositions that provide antiplaque and antigingivitis benefits without staining the teeth.
U.S. Pat. No. 6,479,037 discloses preparing a chlorine dioxide composition for tooth whitening wherein the composition is prepared by combining a chlorine dioxide precursor (CDP) portion with an acidulant (ACD) portion. The CDP portion is a solution of metal chlorite at a pH greater than 7. The ACD is acidic, preferably having a pH of 3.0 to 4.5. The CDP is applied to the tooth surface. The ACD is then applied over the CDP to activate the metal chlorite and produce chlorine dioxide. The pH at the contact interface is preferably less than 6 and, most preferably, in the range of about 3.0 to 4.5. Thus, the resulting chlorine dioxide composition on the tooth surface is acidic. Additionally, this method exposes the oral mucosa to possible contact with a highly acidic reagent (ACD).
However, all of the above patents describe the use of compositions and methods that are damaging to biological tissues, including soft tissues and hard tissues, such as tooth enamel and dentin. Furthermore, despite being effective for many different purposes, the unthickened, runny, and liquid consistency of many of these solutions limits the potential uses of the solution and often requires concerted effort from a user to ensure the solution is being applied in an effective manner.
What is needed are compositions and methods for the use of chlorine dioxide, wherein biological tissue is not damaged. Additionally, a thickened chlorine dioxide mixture is needed that has the consistency needed to remain on a surface or substrate for any period of time and the chlorine dioxide concentration needed to be effective thereon without requiring significant concerted effort from the user. The present disclosure meets and addresses these needs.